I too however do prefer OpenVPN but if your requirements are that it works with Windows natively PoPToP is what you're looking for. We use it where I work for two reasons. It's what they were using when I started here and it works with the iPhone where OpenVPN doesn't currently.

Linux's IPSec servers (FreeSWAN, OpenSWAN, StrongSWAN) all support L2TP, which WinXP, Vista (and i presume 7) all support natively. Installing the certificate is a minor PITA, but after that it works like PPTP, only without the nasty vulnerabilities in the latter!